Rebuilt engine spun no. 1 rod main.

[deadlew]

Brief history on the block I had rebuilt. Had this '78 350 4 bolt main block in my FJ40 and it spun no. 1 and no. 2 rod mains. So the 305 in my K5 dies and I send the 350 block, different crank, and different rods out to be machined. The shop bores it .030 over, grinds the crank .010 under on the rods and mains, and assemblys the short block.

I get the short block back, put the rebuilt heads on it, install the cam and lifters and so on. Used assembly lube everywhere, prime the engine before I started it and broke the cam in. Changed the oil after cam break in, 500 miles and again at 1000 miles. About 1500 miles the motor makes an awful noises out of the bottom end. Have it trailered home and pull the pan. No.1 rod main bearing is gone and in pieces in the pan, No. 1 main has a spot worn down to the copper, No. 2 Rod main is scored badly, No. 2 main still has the white film on the bearings. The rest of the rods are tight and in good shape. Could the problem be with the block?

Thank for your insight.

PaulC

 

[454k30]

could it possibly be a blocked oil chanel? Sounds to me like the front end of the engine isnt getting the oil it needs.

 

[reddog64]

I'm machinest but that's what it sounds like to me too..

 

[Hogback Fabrication]

**Disclaimer**

This is what I think, I don't have a ton of experience rebuilding motors


I thought that when you spin a bearing it destroys the surfaces on the block and bearing caps. You said you had the crank turned down .010" but was any machine work done on the bearing seats on the block?

I know they make oversized bearings to compensate for a crank that's been turned down but do they make bearings for blocks that have had their bearing journals machined?

 

[4X4HIGH]

Are we talking about a spun main bearing or rod bearing? If a main bearing spun then the block needed to be align bored or align honed. If this wasn't done then a repeat failure is in short order.

 

[MaxPF]

Did the bearings actually spin, or just burn up? A loss of oil to the bearings can result in either scenario. Spun bearings are usually a result of too little clearance or insufficient crush.

I agree that it looks like an oiling problem. The affected rods go bye-bye first, with the related main chasing it to oblivion. What doesn't make sense is the destruction of #2 rod bearing with no destruction of of #3 RB or #2 main - both of those should have been damaged as well since they are in the same oil path. I wonder if this was a bearing fitment problem on the #1 main and combined #1 and 2 rod journals?

The others are right about the block needing to be re-registered and align bored if it has spun a main bearing. Otherwise the bearing bore will certainly be too loose and the replacement bearing will not have enough crush.

 

[deadlew]

No. 1 Rod bearing spun on the crank and was small pieces of scrap metal in the pan. No. 1 Crank bearing had a spot worn down to the copper already.

PaulC

 

[3 on the tree]

My experience indicates a line bore of the block, if a different crank is used. Also, core shift can occur when a block sits bare long enough.

Back in the 80's, Pontiac had a real problem with their 301 blocks. I have seen them shift so much, a crank would not drop in!!!

 

[blazinzuk]

It is more likely that the above answers are correct. I had a machine shop turn down a crank for me and it spun no 1. As always they claimed it was my fault I assembled something wrong. Took it to another machine shop, the crank journal was machined in an oval shape not round. Got another crank threw it in and no problems.